What’s more annoying to a mobile-phone user than not having service? If you answered “nothing,” you’re right. And never is that truer than when a user is trying to share something. Like the awesome photo they just snapped from their courtside or ringside seats as the … puck hit a … home run … through the fullback’s legs.

Okay, I don’t do sports. But according to my basketball-savvy friends, we’re in the midst of something called the “2012 playoff season,” which I gather is a pretty big deal. In an effort to avoid butchering any sports terminology, I will put this succinctly: The Boston Celtics will be playing basketball against the Philadelphia 76ers tonight at Boston’s TD Garden. That means the sports arena, pictured in the title image, will in a few hours fill with close to 19,000 people.

By any measure, that’s a lot of humans under one roof. To a wireless carrier, though, that kind of situation is mayhem. Effectively providing service to even half that number of users, employing only Boston’s existing cellular networks, is a recipe for failure. Most of us have experience with cell phone service in the midst of massive groups of people: if it doesn’t disappear, it at least degrades to nearly unusable levels. That’s because wireless networks have traditionally been designed to provide uniform coverage across a large area. When many users congregate in one relatively small sector -like a sports arena or concert hall- a variety of capacity issues conspire to make communication with the network almost impossible.

Click to expand, and observe the trials and tribulations of Bob the Stick Man.

To combat these issues and ensure its customers enjoy more reliable service at events hosted by TD Garden, AT&T has installed a Distributed Antenna System (DAS) inside the building. This network of over a hundred miniature cell sites completely blankets the arena in cellular coverage, which AT&T hopes will ensure a seamless, smooth experience for its users’ texts, tweets, and picture messages. This morning, AT&T and TD Garden representatives allowed some press folks, myself included, a peek behind the curtain to see the guts of the network enhancements.

AT&T has been heavily promoting its new 4G LTE network alongside its older UMTS system as it continues its heated battle with Verizon for the title of the nation’s biggest 4G head-honcho. The new Distributed Antenna System within TD Garden reflects this; in addition to standard UMTS “faux-G” coverage, the in-building network also provides full 4G LTE support.

The Garden’s DAS is divided into ten sectors each for UMTS 3G and LTE, though the capability exists to expand to twelve sectors in the future, should it become necessary. Right now, AT&T is the only carrier making use of the DAS, but expansion space is available for other wireless companies, should they choose to move in.

The antenna panels themselves are mounted in pairs on the Garden’s suite level, custom-designed to blend in with existing ceiling-mounted appliances like flatscreen televisions and electronic billboards. In person, I was impressed by how small the units were; each matte black antenna “pod” isn’t much bigger than a portable cooler. Had they not been pointed out, I wouldn’t have been likely to notice them, and if I had, I’d have taken them for speakers or something similarly mundane. They’re very low-profile.

Don’t mind us; we’re just tossing out some awesome coverage.

Those are just the “big ones,” though; higher up, on the balcony level, connectivity is provided by antennas mounted above the ceiling tiles. Working in concert with their exposed counterparts below, these antennas generate a large enough footprint to completely immerse nearly every corner of the arena in AT&T’s LTE and UMTS coverage. Since all of the base stations are mounted along the building’s perimeter, I asked what the reception was like down on the court floor itself. I reasoned that a phone sitting in the middle of the center circle would struggle to prioritize, listening for the strongest of many signals coming in from all directions. But I was assured that, though AT&T’s engineers were still doing some tweaking, if Ray Allen wanted to send a text or tweet a picture in the middle of a layup, it would indeed be possible.

Whether or not this would result in a fine from the NBA remained an open question.

Coverage in the “bowl” itself, though, is only part of the story; remember, in a building this size, there are still thousands of feet of corridors connecting concession stands, luxury suites, and restrooms – not to mention all of the service passageways and control rooms for the employees who keep the place humming. AT&T didn’t neglect these zones, installing hundreds of additional antennas on and above ceiling panels everywhere, to provide as seamless a coverage blanket as possible. When we asked why some antennas were mounted in plain view, while others were hidden behind the tiles, an AT&T official reminded us how rowdy sports fans can get at times. It was then that I noticed how low some of the ceilings were, and instantly understood why some aerials were hidden safely out of sight.

Tempting targets.

We performed a couple speed tests in the midst of our tour, just for curiosity’s sake; of course, the network in its non-loaded condition was virtually guaranteed to impress us, and that it did. We were consistently achieving upload speeds of 20-25Mbps, though download speeds were only about a quarter that. Another reporter asked if this was due to AT&T attempting to optimize the network performance for uplink, reasoning that more users would be uploading than downloading media at most Garden events, but we were told that it was just a result of the network still undergoing optimization.

That was with an empty stadium and only a handful of users, though; the real stress test will come tonight, when thousands upon thousands of customers start hammering out MMS messages, tweets, and Facebook posts. “Stress test” doesn’t quite cover it.

LTE equipment in the foreground, UMTS in the back. All the little elves inside are pumping iron, and if you listen closely, you can hear the Rocky theme blaring from tiny boom boxes.

All of this is no small feat. The interior of the TD Garden, like other high-capacity venues, is an engineer’s nightmare; RF-opaque materials like steel and concrete are arranged in jagged layouts heavy on right angles. The challenge of dealing with destructive interference from multipath effects -and other signal attenuation issues far beyond my comprehension- must be enormous.

And that’s what was on my mind as the tour wrapped up this morning; not the subtly hidden antennas or racks of humming radio equipment, but the people who’d brought it all together. A small team of engineers was on hand throughout the morning, and watching them answer questions about the network they’d built was more fascinating than any blinking cabinet lights or speed test battery. Of course there was the competence and calm certitude that such a mentally challenging occupation demands, but there was also a palpable sense of pride in their accomplishment. It wasn’t arrogance or vanity; just the quiet satisfaction of having created something powerful and invisible that thousands of people would soon depend on. After so many years of talking about wireless networks in the abstract -and usually in a decidedly uncomplimentary sense- it was gratifying to be reminded that there were humans behind the magic, and that they took pride in their work.

So tonight, as the Celtics beat up on the 76ers (something I’m contractually obligated to say, as a Bostonian), I won’t be rooting as much for the ball players as the engineers, whose creation will for the first time light up with the traffic it was so carefully designed to carry.